Advances in Earth Science ›› 2005, Vol. 20 ›› Issue (2): 149-157. doi: 10.11867/j.issn.1001-8166.2005.02.0149

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WANG Zhaoyin 1; GUO Yanbiao 2; LI Changzhi 1; WANG Feixin 1   

  1. 1. Department of Hydraulic Engineering, Tsinghua University, Beijing 100084, China;2. College of Resources and Environment, South China Agricultural University, Guangzhou 510642, China
  • Received:2003-06-23 Revised:2004-04-02 Online:2005-02-25 Published:2005-02-25

WANG Zhaoyin;GUO Yanbiao;LI Changzhi;WANG Feixin. VEGETATION-EROSION CHART AND ITS APPLICATION IN TYPICAL WATERSHED IN CHINA[J]. Advances in Earth Science, 2005, 20(2): 149-157.

    This paper studies the development of vegetation and erosion of typical watersheds in China employing the vegetation-erosion dynamics:
The four parameters a, c, d, f in the couple differential equations are determined by using the data of vegetation-erosion process within the watersheds. The parameters are functions of climatic, topographical and soil conditions and independent of the vegetation and erosion, ecological stresses and human activities. With these parameters we developed the vegetationerosion chart to predict the developing trend of vegetation and erosion. The vegetation of a watershed may exists in four states: vegetation developing and erosion reducing (C zone in the chart), vegetation deteriorating and erosion increasing (A zone), both vegetation and erosion increasing (B zone) and both vegetation and erosion decreasing (D zone).
    The vegetation-erosion chart for a watershed is exclusively given by the parameters a, c, d, fThe parameters are constant and universal for the areas of the same climate, topography, soil and vegetation compositions. If the values of the parameters a and f are large, the system in most cases is in the C-Zone developing toward perfect vegetation. If the values of c and d are large, the system in most cases is in the A-Zone developing toward poor vegetation and high rate of erosion.
    We worked out the vegetation-erosion charts for the Wangjiagou, Anjiagou, Xiaojiang watersheds and Xishan areas with the determined values of a, c, d, f from data. The erosion rate in Xiaojiang watershed is extremely high and the vegetation cover is relatively poor although the area is wetter and warmer than north China. It is still in Zone A and exhibits the development trend toward poor vegetation, which can offset, to a certain extent, the human's effort for improvement of the vegetation. The dashed curve in the figure shows the suggested route for moving the system into the C-zone. First, erosion control must be performed to reduce the erosion rate by 60%-80%, thus the system is moved into Zone B. Then the development trend V'>0 may support human's reforestation project moving the system into Zone C. Erosion control is very important in the area for re-vegetating the hills. Mere planting trees and shrubs do not work well for greening the high erosion land. 
    For the Wangjiagou and Anjiagou Watersheds on the loess plateau, however, there is no Zone B but Zone D, in which V'<0,E'<0. The ecological system in the D-zone is also unstable, because both vegetation and erosion are reducing. If the vegetation reduces faster than erosion the system may enter into the A-zone and if erosion reduces faster than vegetation the system may enter into the C-zone. The C-zone in the charts for the two watersheds is very small and the D-zone is large. The tangent of the line E'=0 is high, implying high effectiveness of which vegetation may control erosion. On the other hand, the line V'=0 is almost horizontal, which suggests that erosion control helps little for vegetation development. Thus, in the loess plateau the most effective way to control erosion is to increase the vegetation cover, so to move the system from the A-zone into the D-zone. Thence the system will automatically move in the direction of erosion reduction but also vegetation reduction. Human can further increase the vegetation cover by planting trees. The dashed curve shows the route of the watershed management and vegetation development. Unfortunately, the C-zone is too small for stable vegetation development. Human effort is always needed to maintain a stable or increasing vegetation cover. For the two small watersheds, although reforestation and erosion control have been continued for more than 30 years, the rate of erosion has been reduced by 90% and the vegetation cover has increased by 8-12 times, the state of the systems is still in the D-Zone. It is quite possible for the system to return to Zone A if the vegetation is not carefully protected. To move the system into the C-Zone, both reforestation and erosion control must be continued.
    In summary, the vegetation-erosion charts demonstrate that on the Yunnan Plateau, the precipitation and temperature are high; vegetation can develop well if erosion is controlled and the vegetation is stable after improvement. On the dry and cold Loess Plateau, vegetation can effectively control erosion but erosion reduction exhibits low effectiveness on vegetation development. Vegetation in the area is not stable and management is always needed to maintain the vegetation. The Xishan area in the outskirts of Beijing is not so wet and dry, not so cold and warm, the vegetation-erosion chart illustrates that the development trend of vegetation is between the two extreme cases. The C-zone is not so big as the Xiaojiang watershed but much larger than the loess plateau, which implies a relatively stable vegetation may be maintained. Reforestation is the most effective measure to control erosion and improve vegetation in the area because the slope of the line E'=0 is very steep in the chart.

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